Cytochrome b5 is the terminal electron donor responsible for maintaining hemoglobin in its reduced functional state. Recently, we have found evidence that suggests that changes in the backbone dynamics of cytochrome b5 may facilitate this physiological role. We have measured the temperature dependence of the reduction potential and using a van't Hoff like analysis of the data, we find that the entropy change associated with oxidation is positive. We have also found, based on measurements of deuterium exchange kinetics, that large scale changes in the strengths of hydrogen bonds occur throughout the molecule upon oxidation. Preliminary measurements of differences in backbone dynamics in reduced and oxidized states of the protein are also consistent with this hypothesis. Through more detailed analyses of field dependent 15N relaxation data we hope to be able to quantitatively relate the contribution of backbone dynamics to the entropy change associated with oxidation using relationships developed by Akke, et al. (1993).